Solder alternatives for the transition to flexible, integrated electronics
By IDTechEx, Cambridge, UKElectronics Production / Materials Engineering Printable Electronics Supply Chain adhesives conformal Flexible pcb pcbs solder substrates surfaces
Conventional lead-free solder not so well suited for emerging applications
Attaching electronic components such as integrated circuits and LEDs to substrates is an essential aspect of electronics manufacturing. While conventional lead-free solder excels at the task for conventional rigid printed circuit boards (pcbs), it is not so well suited for emerging applications that require components to be attached to flexible substrates or conformal surfaces.
Component attachment landscape
Component attachment materials can be divided into two categories, as shown in the graphic. Solder, formerly based on lead but now primarily lead-free, offers the twin advantages of being low-cost and well-established. Furthermore, the reflow process assists with component alignment, with components being reoriented to minimize the surface energy of the solder droplet. This self-alignment significantly reduces the resolution requirements of the pick-and-place process, enabling higher speed component placement.
However, solder has one key disadvantage. The reflow process generally requires high temperatures of around 250 C for widely used SAC (tin/silver/copper) solder, and also requires time-consuming thermal ramps. As such it’s not applicable for thermally fragile substrates such as PET, and nor is it compatible with either high-speed roll-to-roll (R2R) manufacturing or 3D additive electronics. Note that ultra-low temperature solders have recently been developed to improve compatibility with polymeric substrates but have yet to gain widespread acceptance.
Don’t require high reflow temperatures or lengthy thermal ramps
The high temperature and time-consuming reflow process leaves space for electrically conductive adhesives (ECAs), discussed in detail in a new report from IDTechEx’s. Rather than being a metal alloy like solder, ECAs comprise a metal powder embedded within a polymer resin.
Although more expensive and without the benefit of self-alignment, ECAs enable components to be immediately attached to the substrate and don’t require either high reflow temperatures or lengthy thermal ramps. They are thus highly unlikely to replace solder for conventional PCBs, but are predicted to increase their market share with a transition towards integrated and flexible electronics.
Furthermore, anisotropic conductive adhesives ACAs enable a lower pitch capacity than the more common isotropic conductive adhesives (ICAs). At present, ACAs (which are supplied as either films or pastes) require high temperature and pressure to be applied during component placement, increasing both manufacturing costs and the risk of component damage. However, the emerging technology of ‘field-aligned conductive adhesives’ removes this constraint since anisotropic conductivity is induced prior to attachment.
Emerging applications for ECAs
ECAs are very well suited to applications that require electronic components to be mounted on anything other than a conventional FR4 substrate, especially when the substrate material is thermally fragile.
A promising emerging application for ECAs is in-mold electronics (IME), used to make decorative surfaces with integrated capacitive touch sensing and lighting. Automotive interiors are the major use case, with the total IME market forecast to reach around $1.5bn by 2032. IME components are manufactured by screen printing conductive ink onto a flat (usually polycarbonate) substrate, after which electronic components are mounted. ECAs are used as the polycarbonate would not withstand the high solder reflow temperatures.
Another promising ECA application is flexible hybrid electronics (FHE), a developing approach which can be regarded as taking the ‘best-of-both’ from conventional and printed electronics. This requires components to be mounted onto flexible substrates with printed conductive traces, with ECAs (or alternatively ultra-low temperature solder) required so as not to damage low-cost PET substrates.
A very specific use case for ECAs is stretchable electronics, required for e-textiles and some electronic skin patches. This will require slightly stretchable component attachment solutions. Since the mechanical properties of ECAs are largely dependent on the properties of the polymer resin, slight stretchability can be introduced by using an elastomeric material.